Referring to FIG. 1, an indoor unit and an outdoor unit in a microwave system are usually separately installed and connected by an intermediate frequency cable adapted to transmit/receive intermediate frequency signals, transmit/receive control signals, and supply power. For a large capacity microwave system, the performance indexes of an intermediate frequency receiving circuit have direct impact on the sensitivity of the system, and accordingly, the significance of the intermediate frequency receiving circuit is obvious.
The critical indexes of the intermediate frequency receiving circuit include reflection factor, in-band flatness, suppressing degree of the transmitted signals and control signals, dynamic range, group delay characteristics, and the like.
As for an intermediate frequency receiving channel of the indoor unit in the microwave system, referring to FIG. 2, a prior intermediate frequency receiving circuit generally performs band-pass filtering first, then amplification and filtering, analog-to-digital conversion sampling, and finally digital demodulation.
During the research, the inventor of this application discovers that although the prior intermediate frequency receiving circuit satisfies the functional requirements, it is unable to provide desired intermediate frequency receiving channel indexes. In particular, the prior intermediate frequency receiving circuit at least has the following disadvantages in terms of the performance indexes:
(1) The in-band unflatness index is not ideal. The prior intermediate frequency receiving circuit separates the receiving circuit from other circuits by a band-pass filter. For a large capacity microwave, the bandwidth for transmitting/receiving the intermediate frequency signals is up to 28 MHz, and the 3 dB bandwidth of the filter becomes even larger when the in-band unflatness is less than 1 dB. Meanwhile, the frequencies of transmitting and receiving the intermediate frequencies are only separated by little more than 200 MHz, so that a steep transition band of the filter becomes necessary in order to reduce the interference between the transmitting/receiving channels. However, the steep transition band of the band-pass filter contradicts the in-band flatness (the condition of the wideband), and the two are difficult to become optimal at the same time. For example, the prior intermediate frequency receiving circuit has in-band unflatness lower than 1 dB.
(2) The improvement of the reflection factor is little. As for the wideband, in the prior intermediate frequency receiving circuit, due to the wideband characteristics assumed by the input impedance of the amplifier and influences from an intermediate frequency port protection circuit, it is difficult to improve the reflection factor by adjusting the band-pass filter of the receiving channel without affecting the in-band flatness of the band-pass filter, and the discreteness is also large. For example, the reflection factor of the port in the prior intermediate frequency receiving circuit is generally −15 dB.
(3) The receiving sensitivity of the system is not high. For example, the prior intermediate frequency receiving circuit generally has a receiving sensitivity of about −69 dBm in a 128 quadrature amplitude modulation (128 QAM) mode.
(4) The suppressing of the transmitted intermediate frequency signals and low frequency control signals is not high and generally around 60 dB.